Process for making lysine-glutamic acid dipeptide derivatives

ABSTRACT

The invention relates to compounds of the formula 
     
       
         
         
             
             
         
       
         
         
           
             and to a process for making same and to the use of the products in the solid phase peptide synthesis. The compounds of formula I are versatile peptide intermediates for the solid phase peptide synthesis (SPPS) of peptide drugs which comprise a Glu-fatty alkyl side chain building block attached to a Lys-part of the peptide chain.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/348,093 filed on Nov. 10, 2016, which is a divisional of U.S.application Ser. No. 14/539,126 filed on Nov. 12, 2014, which is acontinuation of PCT/EP2013/059759 filed on May 13, 2013, which claimspriority to EP Patent Application No. 12168119.1 filed on May 15, 2012,the disclosures of which are all incorporated herein by reference intheir entirety

SUMMARY OF THE INVENTION

The invention relates to compounds of the formula

wherein,

R¹ and R² are the same or different and denote hydrogen or an esterprotecting group,

R³ is hydrogen or an amino protecting group and

R⁴ is C₁₂₋₂₀-alkyl,

and its enantiomers, diastereomers and salts.

The invention in a further embodiment relates to a process for thepreparation of the compounds of the formula I and to the use of thecompounds of formula I in the solid phase peptide synthesis.

The compounds of the present invention have been found to be versatilepeptide intermediates for the solid phase peptide synthesis (SPPS) ofpeptide drugs which comprise a Glu-fatty alkyl side chain building blockattached to a Lys-part of the peptide chain. For example Liraglutide canbe mentioned, which is a GLP-1 analog diabetes-2 peptide drug.Liraglutide carries a Glu-hexadecanoyl side chain building block at theLys²⁶ position (Wikipedia, the free encyclopedia of 30.04.2012)

An object of the present invention is to provide novel peptideintermediates which carry a Glu-fatty alkyl side chain and which canreadily be inserted in the SPPS. It was found that the compounds of thepresent invention of the formula

wherein,

-   R¹ and R² are the same or different and denote hydrogen or an ester    protecting group,-   R³ is hydrogen or an amino protecting group and-   R⁴ is C₁₂₋₂₀-alkyl,-   and its enantiomers, diastereomers and salts have the potential to    very well serve this purpose.

DETAILED DESCRIPTION OF THE INVENTION

The term “C₁₂₋₂₀ alkyl” used for substituent R⁴ refers to a branched orstraight-chain monovalent saturated aliphatic hydrocarbon radical oftwelve to twenty carbon atoms, particularly to a straight-chainmonovalent saturated aliphatic hydrocarbon radical. The term can beexemplified by the radicals dodecyl, tridecyl, tetradecyl, pentadecyl,hexadecyl, heptadecyl, octadecyl, nonadecyl and eicosanyl.

In a particular embodiment R⁴ refers to C₁₄₋₁₆ alkyl, even moreparticularly to C₁₅ alkyl.

More particularly R⁴ is tetradecyl, pentadecyl or hexadecyl, butparticularly pentadecyl.

The term “amino protecting group” used for substituent R³ refers tocommon substituents conventionally used to hinder the reactivity of theamino group. Suitable amino protecting groups are described in “FmocSolid Phase Peptide Synthesis—A Practical Approach” W. C. Chan & P. D.White, Oxford University Press, 2000, reprinted 2004, printed digitally.

Particularly R³ is Fmoc (9H-fluoren-9-ylmethoxycarbonyl).

The term “ester protecting group” used for substituents R¹ and R² refersto any substituents conventionally used to hinder the reactivity of thehydroxy group. Suitable hydroxy protecting groups are described in GreenT., “Protective Groups in Organic Synthesis”, Chapter 1, John Wiley andSons, Inc., 1991, 10-142 and can be selected from C₁₋₄-alkyl, optionallysubstituted with phenyl, C₂₋₄-alkenyl, piperidinyl ordimethylaminoboranyl. Particular ester protecting groups for R¹ and R²are C₁₋₄-alkyl or C₂₋₄-alkenyl.

More particularly R¹ is t-butyl and R² is allyl.

The term “salts” in connection with the compounds of the presentinvention embrace the customary salts the skilled in the art wouldapply, such as hydrochlorides, acetates, trifluoroacetates or formiates.In a particular embodiment of the invention R¹ is hydrogen or C₁₋₄-alkyland R² is hydrogen or C₂₋₄-alkenyl. In another more particularembodiment of the invention R¹ is t-butyl and R² is hydrogen or allyl.In a particular embodiment of the invention the compounds of formula Ihave the formula

wherein,

R¹, R², R³ and R⁴ are as above and its enantiomers, diastereomers andsalts. The compounds of formula Ia or Ib with the substitution patternas of below are even more particular embodiments of the invention:

-   -   R¹ t-butyl, R² hydrogen, R³ Fmoc, R⁴ C₁₅-alkyl, particularly        pentadecyl.    -   R¹ t-butyl, R² allyl, R³ Fmoc, R⁴ C₁₅-alkyl, particularly        pentadecyl.        In a more particular embodiment the compounds of formula I have        the formula Ia. The compounds of the present invention can be        prepared with processes which in principle are known to the        skilled in the art of peptide synthesis.

For the preparation of compounds of formula I, wherein R² is hydrogen,the process comprises

a) coupling the glutamic acid derivative of formula

wherein R¹ and R⁴ are as above, or a salt thereof with a lysinederivative of formula

wherein R²′ is an ester protecting group and R³ is as above, or a saltthereof to form a compound of the formula

wherein R¹, R^(2′), R³ and R⁴ are as above and

b) removing the ester protecting group R^(2′).

Step a)

Step a) requires the coupling of the glutamic acid derivative of formulaII with the lysine derivative of formula III.

The glutamic acid derivative of formula II can be prepared following thescheme 1 below starting from commercially available starting materials.

A suitable commercially available glutamic acid derivative of formula IIis the (S)-5-benzyl 1-tert-butyl 2-amino-pentanedioate hydrochloride.

The lysine derivatives of formula III are commercially available.Suitably the (S)-allyl6-amino-2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-aminohexanoate isused.

The coupling of the glutamic acid derivative of formula II with thelysine derivative of formula III can then be performed applying theclassical techniques for peptide synthesis.

Accordingly the glutamic acid derivative of formula II is initiallyactivated with an activating agent which is customary in the art such aswith carbonyldiimidazole (CDI), carbodiimides selected from e.g.dicyclohexylcarbodiimide (DCC) or diisoopropylcarbodiimide (DIC) ortriazols selected e.g. from 1-hydroxy-benzotriazole (HOBt) or1-hydroxy-7-aza-benzotriazole (HOAt).

Good results have been achieved with CDI (1,1′-carbonyldiimidazole)applied in a suitable organic solvent, like e.g. dichloromethane.

The coupling then can place with the lysine derivative of formula III inthe presence of an organic base such as triethylamine, as a rule at roomtemperature.

The resulting dipeptide compound of formula Ib can be obtained from theorganic phase by evaporation of the solvent and subsequentcrystallization of the residue in a suitable organic solvent, such as indiethyl ether.

The compounds of formula Ib as subgenus of formula Ia outlined above,are particular embodiments of the present invention.

Particular representatives of compounds of formula Ib are (S)-allyl2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoatewith R¹=t-butyl, R^(2′)=allyl, R³=Fmoc and R⁴=pentadecyl.

Step b)

Step b) requires the removal of the ester protecting group R^(2′) toform the compound of formula Ia.

This reaction is well known to the skilled in the art.

A suitable system for removing the allyl group is for instance asolution of a Pd-source, like tetrakis(triphenylphosphine)palladium(0)and of phenylsilane in an organic solvent such as dichloromethane,tetrahydrofuran or methyl tetrahydrofuran.

The reaction can take place at room temperature.

The resulting dipeptide of formula Ia can be obtained from the organicphase by evaporation of the solvent and subsequent digestion of thecrude product with a suitable organic solvent such as with heptaneand/or a mixture of heptane/dichloromethane.

As outlined above, the compounds of formula I can be used as versatileintermediates in the solid phase peptide synthesis, particularly in thesynthesis of peptides which comprise a Glu-fatty alkyl side chainbuilding block attached to a Lys-part of the peptide chain.

Even more particularly the compounds of formula I can be used in theFMOC solid phase peptide synthesis of such peptides.

EXAMPLES Abbreviations:

r.t.=room temperature, DCM=dichloromethane, THF=tetrahydrofuran,TBME=tert.-butyl methyl ether, EtOAc=ethyl acetate, TLC=thin layerchromatography

Example 1(S)-2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoicacid a) (S)-5 -benzyl 1-tert-butyl 2-palmitamidopentanedioate

In a 200-mL, 3-necked flask, a mixture of (S)-5-benzyl 1-tert-butyl2-amino-pentanedioate hydrochloride (5.00 g, 14.9 mmol), triethylamine(3.12 g, 30.7 mmol) and tetrahydrofuran (100 mL) was stirred at 0-5° C.for 15 min. To the suspension, palmitoyl chloride (4.35 g, 15.5 mmol)was added via a syringe within 10 min. The reaction mixture was stirredfor additional 30 min at 0-5° C. As to TLC (EE/heptane 1:1, R_(F)starting material=0.1, R_(F) product=0.6, detected with aid ofKomarowsky's reagent at 254 nm (cf. P. Stevens, J. Chromatog. 1964, 14,269)) the conversion was complete. To the reaction mixture, water (60mL) and tert-butyl methyl ether (70 mL) was added and the mixture wasstirred at r.t.

for 5 min. The organic layer was separated, washed with brine (120 mL),dried over sodium sulphate and evaporate to dryness to afford(S)-5-benzyl 1-tert-butyl 2-palmitamidopentanedioate (8.21 g, >99%) as awhite solid with 98.9% chemical purity (LC method see below).

M.p. 47° C.; EI-MS: m/z=531.39 (M+H)⁺.

LC method: X-Bridge phenyl column No. 823, 50×4.6 mm, ID 2.5 μm; mobilephase, A: water/NCMe (95:5), B: NCMe, C: water/glycine (pH 9); flow: 3ml/min; gradient from 50/4/55 (A/B/C) to 7/88/5 (A/B/C) within 2 min,isocratic 7/88/5 (A/B/C) for 0.8 min. Retention times: 0.54 min ((S)-and (R)-5-benzyl 1-tert-butyl 2-amino-pentanedioate), 2.17 min ((S)- and(R)-5-benzyl 1-tert-butyl 2-palmitamidopentanedioate).

b) (S)-5-tert-butoxy-5-oxo-4-palmitamidopentanoic acid

In 250-mL 3-necked flask, a mixture of crude (S)-5-benzyl 1-tert-butyl2-palmitamidopentanedioate (13.2 g, 24.8 mmol), 10% palladium oncharcoal (1.31 g, 1.20 mmol) and THF (150 mL) was stirred under hydrogenatmosphere at room temperature. As to TLC (EE/heptane 1:1, R_(F)starting material=0.5, R_(F) product=0.2, detected with aid ofKomarowsky's reagent (cf. P. Stevens, J. Chromatog. 1964, 14, 269)),after 23 h the conversion was complete. The black suspension was passedthrough a fiberglass filter and the resulting colourless filtrate wasevaporated to dryness to afford the crude product (11.3 g) which wasthen purified via crystallization from heptane to yield(S)-5-tert-butoxy-5-oxo-4-palmitamidopentanoic acid (8.78 g, 76% yield)as a white solid with 97.7% chemical purity (LC method see below).

M.p. 63° C.; EI-MS: m/z=440.33

LC method: X-Bridge phenyl column No. 823, 50×4.6 mm, ID 2.5 μm; mobilephase, A: water/NCMe (95:5), B: NCMe, C: water/glycine (pH 9); flow: 3ml/min; gradient from 50/4/55 (A/B/C) to 7/88/5 (A/B/C) within 2 min,isocratic 7/88/5 (A/B/C) for 0.8 min. Retention times: 0.77 min ((S)-and (R)-5-tert-butoxy-5-oxo-4-palmitamidopentanoic acid), 2.17 min ((S)-and (R)-5-benzyl 1-tert-butyl 2-palmitamidopentanedioate).

c) (S)-allyl2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoate

In a 500-mL 3-necked flask, a mixture of(S)-5-tert-butoxy-5-oxo-4-palmitamidopentanoic acid (8.77 g, 19.4 mmol),1,1′-carbonyldiimidazole (3.30 g, 20.4 mmol) and DCM (125 mL) wasstirred at room temperature for 90 min. To the resulting whitesuspension, a solution of (S)-allyl6-amino-2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-aminohexanoate(8.68 g, 19.5 mmol) and triethylamine (1.96 g, 19.4 mmol) in DCM (50 mL)was added within 15 min. The reaction mixture was stirred for another 90min at r.t. to complete the conversion (determined via TLC (EE/heptane1:1, R_(F) starting material=0, R_(F) product=0.5, detected with aid ofKomarowsky's reagent (cf. P. Stevens, J. Chromatog. 1964, 14, 269)).Next, DCM (50 mL) and water (40 mL) was added to the mixture and thelayers were separated. The aqueous layer was extracted with DCM (20 mL)and the combined organic layers dried over sodium sulphate. Afterevaporation off the solvent, the residual crude product (16.0 g) waspurified by crystallization from diethyl ether to afford (S)-allyl2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoate(15.2 g, 91%) as white solid with 96.5% chemical purity (LC method seebelow) and >99.9% enantio- and diastereomeric purity (chiral LC methodsee below).

M.p. 118° C.; EI-MS: m/z=832.55 (M+H)⁺.

LC method: X-Bridge phenyl column, 50×4.6 mm, ID 2.5 μm; mobile phase,A: water/NCMe (95:5), B: NCMe, C: 0.1% formic acid in water; flow: 2ml/min; gradient from 65/25/10 (A/B/C) to 10/80/10 (A/B/C) within 10min, isocratic 10/80/10 (A/B/C) for 2 min. Retention times: 9.59 min((S)-allyl2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoate)).

Chiral LC method: Chiracel OD-RH columns No. 745 & No. 702, 150×4.6 mm,ID 5 □m; mobile phase, A: NCMe, B: water/HClO₄ (pH 2); flow: 1 ml/min,isocratic 68:32 (A/B) for 32 min, gradient from 68/32 (A/B) to 75/25(A/B) within 0.5 min, isocratic 75/25 (A/B) for 29.5 min. Retentiontimes: 45.39 min ((R)-allyl2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((R)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoate),47.75 min ((R)-allyl2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoate),51.98 min ((S)-allyl2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((R)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoate),55.66 min ((S)-allyl2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoate).

Example 2

(S)-2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoicacid

In a 500-mL 3-necked flask, a mixture of (S)-allyl2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoate(10.0 g, 11.4 mmol), phenylsilane (7.02 g, 62.9 mmol),tetrakis(triphenylphosphine) palladium(0) (1.00 g, 0.85 mmol) and DCM(250 mL) was stirred at room temperature. As to TLC (EE/heptane 3:1,R_(F) starting material=0.2, R_(F) product=0, detected with UV at 254nm), after 11 min the conversion was complete. The reaction mixture wasdiluted with DCM (50 mL) and washed successively with water (50 mL),aqueous sodium diethyldithiocarbamate (0.5%, 30 mL) and brine (30 mL),dried over sodium sulphate and rotatory evaporated to dryness. Digestionof the residual crude product first with heptane (25 mL) and afterwardswith heptane/DCM (9:1) at at r.t. afforded after filtration and dryingcrude(S)-2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoicacid (8.92 g) with 77.2% chemical purity (LC method see below). Thecrude product contained 11% of triphenylphosphine oxide as majorimpurity. Preparative supercritical fluid chromatography (SFC, methodsee below) of a 1 g sample of the crude product afforded pure(S)-2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoicacid (0.75 g, 72%) as a white solid with 96.7% chemical purity (LCmethod see below), 98.0% enantiomeric and 99.8% diastereomeric purity(chiral LC method see below)

M.p. 119° C.; EI-MS: m/z=792.52 (M+H)⁺.

LC method: X-Bridge phenyl column No. 823, 50×4.6 mm, ID 2.5 μm; mobilephase, A: water/NCMe (95:5), B: NCMe, C: 0.1% formic acid in water;flow: 2 ml/min; gradient from 65/25/10 (A/B/C) to 10/80/10 (A/B/C)within 10 min, isocratic 10/80/10 (A/B/C) for 2 min. Retention times:8.65 min((S)-2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoicacid), 9.59 min ((S)-allyl2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoate)).

Chiral LC method: Chiracel OD-RH columns No. 745 & No. 702, 150×4.6 mm,ID 5 □m; mobile phase, A: NCMe, B: water/HClO₄ (pH 2); flow: 1 ml/min,isocratic 68:32 (A/B) for 32 min, gradient from 68/32 (A/B) to 75/25(A/B) within 0.5 min, isocratic 75/25 (A/B) for 29.5 min. Retentiontimes: 21.56 min((R)-2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((R)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoicacid), 23.52 min((R)-2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoicacid), 25.68 min((S)-2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((R)-5 -tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoic acid), 28.32 min((S)-2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoicacid)

Preparative SFC method: Viridis 2-ethylpyridine OBD column, 150×30 mm,ID 5 □m; 50° C. column temperature; mobile phase, A: CO₂, B: MeOH; flow:60 ml/min, gradient from 80:20 (A/B) to 60/40 (A/B) within 10 min.

Example 3(S)-2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoicacid

In a 250-mL 3-necked flask, a mixture of (S)-allyl2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoate(12.0 g, 13.7 mmol), phenylsilane (2.28 g, 20.4 mmol),tetrakis(triphenylphosphine) palladium(0) (96.0 mg, 0.08 mmol) and DCM(120 mL) was stirred at r.t. As to TLC (DCM/MeOH 9:1, R_(F) startingmaterial=0.9, R_(F) product=0.3, detected with UV at 254 nm), after 3 hthe conversion was complete. The reaction mixture was then washedsuccessively with aqueous sodium diethyldithiocarbamate (0.5%, 20 mL)and brine (75 mL), dried over sodium sulphate and rotatory evaporated todryness to yield crude(S)-2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoicacid (11.6 g) with 93.5% chemical purity (LC method see Example2), >99.9% enantiomeric and 99.7% diastereomeric purity (chiral LCmethod see Example 2) containing 1.2% of residual triphenylphosphineoxide. The crude product was then suspended in heptane (230 mL) for 1 hat r.t, the mixture was filtered and the filter cake was washed withheptane (50 mL) to yield(S)-2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5 -tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoic acid (10.9 g, 97% yield) as ayellowish solid with 96.2% chemical purity (LC method see Example2), >99.9% enantiomeric and 99.8% diastereomeric purity (chiral LCmethod see Example 2) containing 0.8% of residual triphenylphosphineoxide.

M.p. 119° C.; EI-MS: m/z=792.52 (M+H)⁺.

Example 4 ((R)-allyl2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoate

In a 25-mL 3-necked flask, a mixture of(S)-5-tert-butoxy-5-oxo-4-palmitamidopentanoic acid (500 mg, 1.12 mmol),1-hydroxybenzotriazole (175 mg, 1.14 mmol), 1,1′ -carbonyldiimidazole(200 mg, 1.23 mmol) and DCM (10 mL) was stirred at room temperature for90 min. To the resulting white suspension, a solution of (R)-allyl6-amino-2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-aminohexanoate(507 mg, 1.12 mmol) and triethylamine (113 mg, 1.12 mmol) in DCM (5 mL)was added within 5 min. The reaction mixture was stirred for another 60min at room temperature to complete the conversion (determined via TLC(DCM/MeOH 95:5, R_(F) starting material=0, R_(F) product=0.2, detectedwith UV at 254 nm). Next, water (10 mL) was added to the mixture and thelayers were separated. The aqueous layer was extracted with DCM (30 mL)and the combined organic layers dried over sodium sulphate. Afterevaporation off the solvent, the residual crude product (983 mg) waspurified by crystallization from diethyl ether to afford (R)-allyl2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoate(686 mg, 70%) as white solid with 94.2% chemical purity (LC method seebelow) and >99.9% enantio- and diastereomeric purity (chiral LC methodsee Example 1c)

M.p. 114° C.; EI-MS: m/z=832.54 (M+H)⁺.

LC method: X-Bridge phenyl column, 50×4.6 mm, ID 2.5 μm; mobile phase,A: water/NCMe (95:5), B: NCMe, C: 0.1% formic acid in water; flow: 2ml/min; gradient from 65/25/10 (A/B/C) to 10/80/10 (A/B/C) within 10min, isocratic 10/80/10 (A/B/C) for 2 min. Retention times: 9.55 min((R)-allyl2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoate)).

Example 5 (R)-2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoic acid

In a 25-mL 3-necked flask, a mixture of (R)-allyl2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoate(675 mg, 0.76 mmol), phenylsilane (351 mg, 3.14 mmol),tetrakis(triphenylphosphine) palladium(0) (20.0 mg, 0.02 mmol) and DCM(7 mL) was stirred at 10° C. As to TLC (DCM/MeOH 95:5, R_(F) startingmaterial=0.8, R_(F) product=0.2, detected with UV at 254 nm), after 25min the conversion was complete. After additional 15 min, the reactionmixture was diluted with DCM (10 mL) and washed successively with water(10 mL), a aqueous of sodium diethyldithiocarbamate (0.5%, 10 mL) andbrine (10 mL). The organic solution was dried over sodium sulphate androtatory evaporated to dryness to yield crude(R)-2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoicacid (631 mg) with 87.4 chemical purity (LC method see below)), >99.9%enantiomeric and 98.8% diastereomeric purity (chiral LC method seeExample 2). The crude product contained 6% of triphenylphosphine oxideas major impurity. Preparative supercritical fluid chromatography (SFC,method see Example 2) of a 603 mg sample of the crude product affordedpure(R)-2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoicacid (348 mg, 59%) as a white solid with 98.7% chemical purity (LCmethod see below), >99.9% enantiomeric and 99.4% diastereomeric purity(chiral LC method see Example 2)

M.p. 125° C.; EI-MS: m/z=792.52 (M+H)⁺.

LC method: X-Bridge phenyl column, 50×4.6 mm, ID 2.5 pm; mobile phase,A: water/NCMe (95:5), B: NCMe, C: 0.1% formic acid in water; flow: 2ml/min; gradient from 65/25/10 (A/B/C) to 10/80/10 (A/B/C) within 10min, isocratic 10/80/10 (A/B/C) for 2 min. Retention times: 8.33 min((R)-2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoicacid), 9.35 min ((R)-allyl2-(((9H-fluoren-9-yl)methoxy)carbonylamino)-6-((S)-5-tert-butoxy-5-oxo-4-palmitamidopentanamido)hexanoate)).

1. A method for solid phase synthesis of a peptide drug, comprisingusing a compound of formula I

as a building block in the solid phase synthesis, wherein, R¹ and R² arethe same or different and denote hydrogen or an ester protecting group,R³ is hydrogen or an amino protecting group and R⁴ is C₁₂₋₂₀-alkyl,and-enantiomers, diastereomers and salts thereof.
 2. The method of claim1, wherein the peptide drug is a GLP-1 analog diabetes type 2 peptidedrug.
 3. The method of claim 2, wherein the peptide drug is liraglutide.4. The method of claim 3, wherein the compound of formula I is locatedat position 26 of the liraglutide.
 7. The method of claim 1 wherein, theester protecting group is selected from C₁₋₄-alkyl, optionallysubstituted with phenyl, C₂₋₄-alkenyl, piperidinyl ordimethylaminoboranyl.
 8. The method of claim 1, wherein R¹ is hydrogenor C₁₋₄-alkyl and R² is hydrogen or C₂₋₄-alkenyl.
 9. The method of claim1, wherein R¹ is t-butyl and R² is hydrogen or allyl.
 10. The method ofclaim 1, wherein R³ is 9H-fluoren-9-ylmethoxycarbonyl.
 11. The method ofclaim 1, wherein R⁴ is C₁₄₋₁₆-alkyl.
 12. A compound of formula I

wherein, R¹ and R² are the same or different and denote hydrogen or anester protecting group, R³ is hydrogen or an amino protecting group andR⁴ is C₁₂₋₂₀-alkyl, and-enantiomers, diastereomers and salts thereof.13. A process for the preparation of compounds of formula I,

and enantiomers and salts thereof, wherein: R¹ is hydrogen or an esterprotecting group; R² is hydrogen; R³ is hydrogen or an amino protectinggroup and R⁴ is C₁₂₋₂₀-alkyl; the process comprising: a) coupling theglutamic acid derivative of formula II

or a salt thereof with a lysine derivative of formula III

wherein R^(2′) is an ester protecting group, or a salt thereof to form acompound of the formula Ic;

and b) removing the ester protecting group R^(2′) to provide thecompound of formula I.